1. Field of the Invention
The present invention relates to methods and systems for magnetic resonance imaging, and in particular to methods and systems wherein magnetic resonance image data are acquired using an echo planar imaging (EPI) pulse sequence.
2. Description of the Prior Art
Most functional magnetic resonance imaging (fMRI) studies are based on the gradient echo EPI (GE-EPI). In general, this technique is known to be more sensitive to changes in the known BOLD (Blood, Oxygen Level Dependent) contrast than spin echo EPI (SE-EPI). On the other hand, SE-EPI is considerably more robust against signal losses or voids caused by magnetic field inhomogeneities, which typically occur in brain regions near air-tissue interfaces, such as the orbito-frontal cortex or the inferior temporal lobes, as described in Bandettini et al., “Spin-Echo and Gradient-Echo EPI of Human Brain Activation Using Bold Contrast—A Comparative-Study at 1.5 T” NMR in Biomedicine 1994; 7(1-2):12-20 and Norris, “Principles of Magnetic Resonance Assessment of Brain Function,” Journal of Magnetic Resonance Imaging 2006; 23(6):794-807. An example is given in FIG. 1 (reproduced from 3. Schwarzbauer et al. “Spin-echo EPI—The Method of Choice For fMRI Of Brain Regions Affected By Magnetic Field Inhomogeneities?,” 2006; Human Brain Mapping, Florence, Italy. p 206, which shows an experimental comparison of the BOLD sensitivity obtained by GE-EPI and SE-EPI. In the GE-EPI data, a compete sensitivity loss is observed in large parts of the orbito-frontal cortex, which is reflected by the zero t-score in this region (cf. cross hair). Conversely, the same region exhibits excellent BOLD sensitivity (t-score: 4.6) in the SE-EPI data. This is because the 180° pulse refocuses the phase dispersion caused by static magnetic field inhomogeneities.
Single shot dual echo EPI is an effective method for interleaving the acquisition of GE and SE images: Following the acquisition of a GE EPI image, a slice-selective 180° refocusing pulse is applied and a second (SE EPI) image is acquired such that the spin echo occurs in the centre of k-space of that image. For most fMRI applications, however, this acquisition scheme is suboptimal for a number of reasons.
The main drawbacks of using the single shot dual echo EPI sequence for fMRI are:    (1) More than a 100% increase in the minimum repetition time (TR), because two images are acquired per slice.    (2) The resulting temporal resolution is often insufficient for event-related fMRI designs.    (3) The acquisition scheme is inefficient as signal losses due to magnetic field inhomogeneity typically only occur in the lower third of the brain.    (4) The resulting time normalized SNR is suboptimal (meaning SNR normalized by the square root of the total acquisition time).    (5) The 180° refocusing pulses lead to a substantial increase in the SAR (specific absorption rate). This may be particularly problematic at high and ultrahigh field strength.